1. Field of the Invention
The present invention relates to a hydraulic drive system for construction machines such as hydraulic excavators, and more particularly, to a hydraulic drive system for construction machines which includes distribution compensating valves for controlling differential pressures across respective flow control valves, and in which a control force in accordance with a differential pressure between the discharge pressure of a hydraulic pump under load-sensing control and the maximum load pressure among a plurality of actuators is applied to each of the distribution compensating valves to thereby set a target value of the differential pressure across the flow control valve.
2. Description of the Prior Art
Recently, in a hydraulic drive system for construction machines, such as hydraulic excavators and cranes, each equipped with a plurality of hydraulic actuators for driving a plurality of driven members, it is customary to control the discharge pressure of a hydraulic pump in response to load pressures or demanded flow rates, and to arrange pressure compensating valves in association with flow control valves for controlling the differential pressures across the flow control valves by the associated pressure compensating valves, so that the supplied flow rates are steadily controlled when simultaneously driving the hydraulic actuators. Load-sensing control is known as a typical example of controlling the discharge pressure of the hydraulic pump in response to the load pressures.
The load-sensing control is to control the discharge rate of a hydraulic pump such that the discharge pressure of the hydraulic pump becomes higher a fixed value than the maximum load pressure among a plurality of hydraulic actuators. This control increases and decreases the discharge rate of the hydraulic pump in response to the load pressures of the hydraulic actuators, thereby permitting economical operation.
Since the discharge rate of the hydraulic pump has an upper limit, i.e., available maximum flow rate, the pump discharge rate will become not enough, when the hydraulic pump reaches the available maximum flow rate in case of simultaneously driving the plural actuators. This is generally known as saturation of the hydraulic pump. If saturation occurs, a hydraulic fluid discharged from the hydraulic pump will flow into the actuator(s) on the lower pressure side in preference to other actuator(s) on the higher pressure side, the latter actuator(s) being hence supplied with the deficient hydraulic fluid. This results in that the plural actuators cannot be driven simultaneously.
To solve the above problem, with a hydraulic drive system as described in DE-A1-3422165 (corresponding to JP-A-60-11706), two drive parts respectively acting in the valve-opening and -closing directions are provided on each pressure compensating valve for controlling the differential pressure across a flow control valve, in place of a spring conventionally provided for setting a target value of the differential pressure across the flow control valve. The discharge pressure of a hydraulic pump is introduced to the drive part acting in the valve-opening direction, and the maximum load pressure among plural actuators is introduced to the drive part acting in the valve-closing direction. This causes a control force in accordance with a differential pressure between the pump discharge pressure and the maximum load pressure to act in the valve-opening direction for setting a target value of the differential pressure across the flow control valve. When saturation of the hydraulic pump occurs in the above arrangement, the differential pressure between the pump discharge pressure and the maximum load pressure is reduced correspondingly. Therefore, the target value of the differential pressure across the flow control valve for each pressure compensating valve is also reduced and the pressure compensating valve associated with the actuator on the lower pressure side is further restricted, so that the hydraulic fluid from the hydraulic pump is prevented from flowing into the actuator on the lower pressure side with preference. This allows the hydraulic fluid from the hydraulic pump to be distributed corresponding to ratio of the demanded flow rates (opening degrees) of the flow control valves and supplied to the plural actuators, thereby permitting appropriate simultaneous drive of the actuators.
Under such an arrangement, the pressure compensating valve eventually offers a function of reliably distributing and supplying the hydraulic fluid from the hydraulic pump to the plural actuators irrespective of any discharge condition of the hydraulic pump. Therefore, that function is called a "distribution compensating" function and the pressure compensating valve is called "a distribution compensating valve" in this description for convenience.
Meanwhile, in the conventional hydraulic drive system as mentioned above, the control force in accordance with the differential pressure between the discharge pressure of the hydraulic pump under load-sensing control and the maximum load pressure among the plural actuators is applied, as the target value of the differential pressure across the flow control valve, to each of the distribution compensating valves. Therefore, provided that all the drive parts have the same pressure receiving area, the degree of the control force applied to the respective distribution compensating valves becomes equal and all the distribution compensating valves give a similar pressure compensating characteristic. During combined operation to simultaneously drive two or more actuators, for example, the proportion of flow rates supplied to the respective actuators, i.e., distribution ratio, is thus uniquely determined dependent on the opening degrees of the flow control valves regardless of various combinations of the actuators simultaneously driven. This leads to a problem that in some type of combined operation, the hydraulic fluid may be distributed overly or insufficiently to one of the actuators, resulting in a reduction of operability and/or working efficiency.
It is an object of the present invention to provide a hydraulic drive system for construction machines which can give individual pressure compensating characteristics to separate distribution compensating valves, and improve operability and/or working efficiency.